Technical Field
The present disclosure relates to a light emitting device and the manufacturing method thereof, in particular to a chip-scale packaging light emitting device including a light emitting diode (LED) semiconductor die which generates electromagnetic radiation while it is in operation.
Description of the Related Art
LEDs are widely used in various applications including traffic lights, backlight units, general lightings, portable devices, automotive lighting and so forth. Generally, an LED semiconductor die is disposed inside a package structure, such as a lead frame, to form a packaged LED device. It may further be disposed and covered by photoluminescent materials, such as phosphors, to form a phosphor-converted white LED device.
An LED device is usually attached to a substrate by a bonding process such as reflow soldering, eutectic bonding, or the like, wherein electric energy can be transmitted through bonding pads of an application substrate so that the LED device generates electromagnetic radiation during operation.
Recent development of a chip-scale packaging (CSP) LED device has attracted more and more attention due to its promising advantages. As a typical example, a white-light CSP LED device is generally composed of a blue-light LED semiconductor die and a packaging structure covering the LED semiconductor die in a compact chip-scale size. In comparison with a plastic leaded chip carrier (PLCC) LED device, a CSP light emitting device shows the following advantages: (1) The material cost is considerably less by eliminating the use of a bonding wire and a lead frame. (2) The thermal resistance between a LED semiconductor die and a mounting substrate, typically a printed circuit board (PCB), is further reduced without using a lead frame in between. Therefore the LED operation temperature is lowered while under the same driving current. In other words, less electrical energy can be consumed to obtain more optical power for a CSP LED device. (3) A lower operation temperature provides a higher LED semiconductor quantum efficiency for a CSP LED device. (4) A much smaller form factor of the light source provides more design flexibility for module-level LED fixtures. (5) A CSP LED device having a small light emitting area more resembles a point source and thus makes the design of secondary optics easier. A compact CSP LED device can be designed to generate small-Etendue light with higher optical intensity that is specified for some projected light applications, such as automobile headlights.
Since a CSP LED device, mainly comprising an LED semiconductor die and a packaging structure covering the LED semiconductor die, does not have gold wires and a surface mount lead frame, it is directly attached onto an application substrate, such as a PCB, so that the electrodes of a flip-chip semiconductor die inside the CSP LED device can be electrically connected to bonding pads of an application substrate. The electrodes of the flip-chip LED semiconductor die serve another important function to transfer and dissipate heat generated during operation of the CSP LED device to the application substrate as well. Because an LED semiconductor die is made of inorganic material and a packaging structure is mostly composed of organic resin material covering the LED semiconductor die, the organic packaging structure can have considerably larger thermal expansion than the inorganic LED semiconductor die during a high temperature reflow soldering or an eutectic bonding process. Especially, the packaging structure can expand more in the vertical direction than the LED semiconductor die; in other words, the packaging structure expands more against an underlying bonding substrate so that it “lifts” the electrodes of the inside LED semiconductor die off the underlying bonding substrate, resulting in a void gap between the electrodes of the LED semiconductor die and the bonding pads of the application substrate during a high temperature soldering/bonding process. Consequently, a CSP LED device fails to be properly bonded onto the substrate, resulting in electrical connection failure. Other failure modes may include higher electrical contact resistance resulting in higher LED power consumption, or higher thermal resistance resulting in poor heat dissipation, all due to poor welding of the electrodes of the semiconductor die to the bonding pads of the substrate. Accordingly, the overall efficiency and reliability of a CSP LED device attached to an application substrate are reduced.
In order to improve the aforementioned problem, a possible solution is to place a thick solder bump, such as a gold-tin bump, underneath electrodes of a CSP LED device so that a bottom surface of a packaging structure of the CSP LED device is elevated to a higher position to form a marginal gap between the bottom surface of the packaging structure and an underneath application substrate. This marginal gap is preserved for thermal expansion of the packaging structure during a subsequent soldering/bonding process Thus, even though the packaging structure of the CSP LED device can still be thermally expanded unavoidably in the vertical direction during soldering, it does not touch the application substrate to force the electrodes of the LED semiconductor die being lifted off from the application substrate. However, adding a thick soldering bump can significantly increase the material cost to fabricate a CSP LED device and reduce the manufacturing yield of a bonding process during application due to misalignment.
In view of this, providing a solution to solve the aforementioned deficiencies is needed to facilitate practical applications using CSP LED devices.